At the present time, the majority of the heads of existing radicular posts or dowels are equipped with a retentive element designed to be as effective as possible. Indeed, when an impression is to be taken of the mouth, and once the root has been prepared and the intraradicular post has been put in place, these retentive elements must maintain the radicular posts in place in the impression or negative and thereby permit withdrawal of the latter from the mouth. However, even if the radicular posts remain in the impression and are thus freed from their root, there is always the possibility that this technical withdrawal maneuver will have caused slight displacement of said posts. Moreover, posts frequently remain stuck in place when impressions are taken in the mouth. This results from the fact that the intraradicular portion of the post in the tooth root offers a large contact surface and thus creates a strong retentive force whereas the contact surface of the head of the post in the impression is relatively small.
In order to facilitate post withdrawal when impressions are taken in the mouth, post manufacturers have equipped the heads of their radicular posts with highly effective retention elements. The resulting flattened heads, ribbed heads, slotted heads and flanged heads are all designed to form a true means of anchorage in the impression compound (so called; i.e., "impression composition" (lit., "dental paste")). If, by accident, the head of a radicular post of one of these existing designs fails to lift out when an impression is taken in the mouth, it is virtually impossible to replace said head in the impression.
Due to [sic] the shape of its head (flattened, ribbed, flanged), the post lies at an angle with respect to its longitudinal axis. It is therefore impossible to determine the position the ribs or flanges were in. Furthermore, the impression compound will have been partially torn away at this point. Prior to taking an impression, numerous practitioners therefore place an index marker on the compound or on the radicular post itself in order to be able to redetermine the correct orientation should the post not lift out from the patient's mouth. Post supports or prefabricated transfer elements have also been described: these are supplementary parts designed to automatically fasten the post during impression taking, to provide precise indication of post position during manipulations thanks to index markings, to eliminate any error in post positioning, and/or to hold the post in place while the impression is taken. In practice, these additional parts [may] consist of a metal spring or slotted tube that fits onto the head of the post (see patent No. FR 1 463 860 NARBONI).